In the computer industry, perhaps the most common type of mass storage device is the disc memory. Such a device includes usually a plurality of rotating discs, each side carrying an oxide recording surface on which are or can be recorded concentric data tracks. Cantilevered arms are attached to a moveable carriage adjacent the rotating disc, and whose position vis-a-vis the disc is electrically controllable. At the free ends of these arms are located transducing heads carried in sliders or pads which are shifted radially across the recording surface by movement of the carriage. Each pad itself is carried by a mounting which resiliently allows the pad to translate axially, pitch and roll relative to the data track motion as the recording surface changes axial position during rotation with respect to its datum due to unavoidable inaccuracies in its flatness and bearing mounting. The mounting is relatively stiff for other degrees of freedom. The head pads are designed to float on a very thin film of air adjacent the recording surfaces, thereby preventing any actual contact and potential damage inflicted on either the medium or the head by the other. A spring-loaded arm presses on each pad to force it toward the disc and hold it at the proper distance therefrom.
Further, it is desirable that the carriage be capable of retracting these pads past the periphery of the discs before stopping disc rotation, to prevent contact between them, and to permit replacement of the discs. Retracting is usually accomplished by means of a cam and a follower arrangement in which the pad loading arm carries either the cam or the follower, and lifts the head as it nears the edge of the disc during the retraction operation. When the carriage is moved radially inward from its retracted position, the cam and follower cooperate to land the pad on its recording surface.
It is at this point where problems now arise. Modern head pads are mounted in a very flexible gimbal spring which furnish little resistance to pitch and roll of the pad so as to allow it to more accurately follow the slight deviations in position of its recording disc. Thus, when the pad lands on the rotating disc, the pad's air bearing surface may not be perfectly parallel to the recording surface. Such a condition may permit a corner of the pad to penetrate the air film, causing each element to scuff and wear the other. In fact, particles may be scratched from either pad or disc which precipitate an avalanche-type failure where particles abraded from these elements build up faster than the filtering system can remove them.
Previously designed head unload mechanisms may employ 2 cam/follower units (U.S. Pat. No. 3,896,495). If the units are not precisely aligned with each other, the flying surface of the pad will pitch causing the problems just mentioned. Typical single cam lift mechanisms have their cams near a side of the head arm, so the followers can be stacked as a single unit. (U.S. Pat. Nos. 3,531,788; 3,579,213; 3,914,791; 3,786,457; and 3,713,121). Such off-center lifting practically guarantees an unpredictable amount of head pitch during lift and land.
There are some designs which avoid this problem. U.S. Pat. No. 3,984,873 teaches a centrally located cam actuated by a follower which operates independently of the head arm position. U.S. Pat. No. 4,017,898 discloses a centrally located lift arm actuated by a bar which moves vertically respective the disc surface. U.S. Pat. No. 4,120,010 shows a device employing a transversely (respective the head arm's long dimension) shiftable hairpin employing a two-stage camming operation to lift the head. '010 does not lift from the center of the head pad, however, and hence has the potential for rolling the head during lift and land.